In 1992 over 1.1 million air purifiers were sold in the United States alone. This market has arisen due to the adverse health effects experienced by a growing number of people from breathing indoor air of poor quality, or indoor air contaminated by pollutants from outdoors. Indeed, four of the top ten health problems in the U.S. are respiratory related: #1 Sinusitis, #5 Allergies, #7 Bronchitis, and #8 Asthma. Nevertheless, less than 2% of the nearly 94 million households currently own an air purifier.
Air purifiers sold to date have been characterized by a number of deficiencies. They either do not produce the quality of air required or they are noisy and expensive to operate (filter replacement, energy, etc.). This lack of quality products has created a clear market need for the introduction of a superior air purifier at a reasonable cost which resolves problems of indoor air quality and its serious health effects.
Very few improvements have been made in either the technology or design of existing air purifiers in recent years. Units currently on the market essentially utilize one of two methods for air purification. One method incorporates mechanical filters which consist of a flat, or pleated, mat of fibers contained in a supporting frame. The second category of air cleaner uses electronic, or electrostatic, technology. Both methods have drawbacks. Mechanical filters, for example, become breeding grounds for bacteria and other germs. In addition, increasing the efficiency of mechanical filters comes at a significant cost. Electronic air cleaners, on the other hand, can become producers of ozone and are limited in their ability to remove all types of impurities from the air.
The important factors in determining the potential health hazard of particulate/gaseous pollutants are:
1. The degree of toxicity of the pollutant. PA1 2. The mass of the pollutant inhaled (amount of exposure). PA1 3. The size of the particle (the smaller the size, the greater is its potential for causing damage because of where it deposits in the respiratory tract). PA1 4. The "state of health" of the respiratory tract (i.e. its ability to purge itself of inhaled particulate pollutants). PA1 5. The nature of the "particulate", especially microorganisms (germs, virus, bacteria, molds, etc.). PA1 1. Low resistance to airflow. PA1 2. No filter media to dispose of. PA1 1. High initial cost. PA1 2. High maintenance. PA1 3. Can produce ozone. PA1 4. Noisy on high setting. PA1 5. Efficiency decreases rapidly as unit gets dirty. PA1 6. Low germ removal. PA1 7. No gaseous pollutant removal. PA1 8. No germicidal effect. PA1 1. Moderate airflow resistance. PA1 2. Moderate initial cost. PA1 3. Easy replacement. PA1 4. Relatively high efficiency. PA1 1. Charged pollutants neutralize the fiber's charge; resulting in loss of efficiency. PA1 2. Above average expense to replace filter. PA1 3. Only moderate germ removal. PA1 4. No gaseous pollutant removal. PA1 5. No germicidal effect. PA1 1. Totally quiet. PA1 2. High efficiency. PA1 3. Provides negative ion enriched air. PA1 4. No filter to change. PA1 5. Minimal maintenance. PA1 6. Low operating cost. PA1 1. Plating of pollutants on room surfaces (walls, etc.). PA1 2. Limited area of coverage. PA1 3. Limited germ removal. PA1 4. Only moderate removal of gaseous pollutants. PA1 5. Slow removal rate. PA1 6. No germicidal effect. PA1 (As above) PA1 (1) How to increase electrode voltage, in order to increase efficiency, without resulting in arcing. PA1 (2) How to prevent electrode arcing due to high humidity. PA1 (3) How to use non-conductive electrodes to achieve #1 & #2 above, without incurring opposite charge buildup on the electrodes due to migrating charges which will neutralize their electrostatic field. And, how to prevent this charge buildup even when there is compression of the filter media due to high airflow. PA1 (4) How to pre-charge particles without: (a) having them collect on the front electrode, (b) their contributing to a charge buildup on the electrodes, and (c) without their building a path of ionization through the filter which would result in a short circuit of the electrodes. PA1 (1) Combine the utilization of an insulated electrode as the first (or front) electrode with; PA1 (2) A conductive electrode as the second (or rear) electrode which sandwiches a fibrous filter; and PA1 (3) Charge incoming particles by ions of the same polarity as the insulated electrode (+ or -).
Basically we can't change the toxicity of particulate or gaseous pollutants. Therefore, in order to reduce the health hazard of indoor air we need to (a) remove as many particulates as possible . . . especially very small (submicron) size particles which are so detrimental healthwise, (b) reduce gaseous pollutants,(c) eliminate airborne micro-organisms (germs, viruses, etc.), and (d) restore a more natural level of negative ions to the indoor environment . . . ions which stimulate the natural clearing of the respiratory tract.
Therefore, the performance of all air purifiers is measured by: the efficiency with which they remove particulate . . . including those of sub-micron size, their effectiveness in removing gaseous pollutants, the amount of clean air they can provide to the user, and their germicidal effect.
Generally speaking, in engineering any air purification system the fundamental performance criteria should be to achieve over 99% particle removal efficiency at very low airflow resistance.
However, as most air purifiers collect pollutants their airflow rate and/or their efficiency decreases significantly. This, in turn, results in dramatically lower overall air cleaning benefit.
Low airflow resistance is important because the typical fan or blower has great difficulty in moving a high volume of air against resistance. Generally, as resistance increases the volume of air moved decreases proportionally. Fans that are capable of moving a large amount of air against a high resistance are (1) significantly more expensive, (2) much noisier and (3) use more energy to operate.
The various existing air cleaners fall into two general categories: Mechanical Filters (media type) and Electronic Air Cleaners.
For media type filters airflow presents a real dilemma: to increase efficiency, the number of fibers in their media has to be increased; but as fibers are added the resistance also increases.
This was the main impetus behind the design of the electronic air cleaner. Since the unit's airstream faces only the leading edges of a series of plates which make up the "electronic cell", there is very little resistance to airflow. Unfortunately, other aspects of this technology have drawbacks which, in overall performance, negate the benefit of low airflow resistance.
A "Hybrid" filter was produced when it was discovered that certain electrical forces can greatly enhance particle removal efficiency of a fiber media. The Lawrence Livermore National Laboratories are credited with providing the scientific verification of this concept. Several versions of what are now termed "electrostatic" filters have appeared on the market, but again, while they represent a step forward, their overall performance has still left much to be desired.
A mechanical filter generally consists of a flat, or pleated, mat of fibers (the "filter media") contained in a supporting frame. This type of filter removes particles from the air passing through it by collecting them as they impact on individual fibers or are too large to pass between fibers. The percentage of particulate trapped determines the filter's overall efficiency, e.g. 4%, 20%, 50%, or 85%, etc. The typical "furnace filter" will be of low resistance (very few fibers) with very low efficiency . . . 4% to 9%. A "hi-tech filter", the HEPA (High Efficiency Particle Arrestor) filter, will be a high resistance filter (many fibers, densely packed) with a high particle removal efficiency (99+%).
Obviously, the smaller the space between the individual fibers, the smaller the size of particle that can be trapped. Unfortunately, as the openings get smaller the resistance to airflow also increases. It now takes much more energy to push the air through the filter. The reason a very open, but inefficient, filter is used in home furnaces is because the furnace's blower would not be able to move the amount of air needed for proper heating, or cooling, against the resistance of a more dense (more efficient) filter. As a high efficiency (HEPA) mechanical filter loads with particulate, resistance increases further while the amount of air passing through the filter decreases dramatically. This results in a significant lowering of overall air cleaning performance/benefit.
______________________________________ ADVANTAGES: DISADVANTAGES: ______________________________________ LOW-EFFICIENCY MECHANICAL FILTERS: 1. Low initial and 1. Low overall particle replacement cost. removal efficiency. 2. Low resistance to air 2. Very low efficiency flow. (virtually none) 3. Easy to install. in the sub-micron size. 4. Disposable. 3. None to low germ removal. 5. Easy replacement. 4. No removal of gaseous pollutants. HIGH EFFICIENCY MECHANICAL FILTERS: 1. High particle removal 1. High initial and efficiency. replacement cost. 2. High resistance to airflow (=noisy blower). 3. Easily clogged by cigarette smoke. 4. Only some germs removed. 5. No gaseous pollutant removal. 6. Airflow rate drops with loading. 7. No germicidal effect. ______________________________________
The second category of air cleaner is that of the electronic, or electrostatic, air cleaner. These are subdivided into two different methods of operation: Powered (electronic) and non-powered (electrostatic).
Powered units draw air in through a front section which electrically charges the incoming particles with a positive charge, and then passes these particles between a series of plates which are alternately positive and ground. The positive particles are repelled away from the positive plates over to the grounded plates where they collect. Because of their very open configuration, such units naturally have a very low resistance to airflow. Non-powered units have a filter media whose plastic fibers are either permanently charged by heating and cooling them in an electric field (electret media), or have the property of becoming electrostatically "charged" by the friction of the air passing over them.
Another type of electrostatic air cleaner is the negative ion generator. These units produce a large quantity of negative ions into the room air which attach themselves to particles in the air and cause them to precipitate out, or to be attracted to nearby grounded surfaces (walls, etc.). Negative ion generators do not have to rely on airflow as part of their cleaning process and are, therefore, totally noiseless in their operation. Unfortunately, many of the charged particles attach to walls and other surfaces near the ion generator . . . resulting in a "dirtying" of these surfaces which is sometimes not easily cleanable and very often requires repainting of the surface.